Haven't seen this personally but I suspect it could be due to the stacks moving simultaneously thing?

In a few cases I've set two or more sub-stacks to head to the same place but when they get there they stop & give a 'path blocked' message instead of joining together like I'd expected them to do.

So if the AI tells those units to all move at once intending them to move one after the other like in RTW/M2TW, it may suffer from them all colliding instead of joining up? (& lacks a routine to check for this?)

In the case that the strategic situation nearby changes & the units suddenly join up & start doing things, I'd say the AI would be reassessing the situation, & giving new orders, the units being near enough to each other to start merging immediately now trigger merging properly or something.